This invention relates to a receiving apparatus and a receiving method in a CDMA communication system. More particularly, the invention relates to a receiving apparatus and a receiving method in a CDMA communication system in which voice code of a prescribed transmission time interval encoded by a voice encoding scheme such as AMR (Adaptive Multi-Rate) is divided into a plurality of classes, the voice code in each class is expressed by a number of bits that conforms to a prescribed bit rate, a check code of a fixed length is attached to voice code of a prescribed class, the voice code of each class is subjected to error-correction encoding processing, and voice code that has undergone error-correction encoding processing in each class is transmitted upon being multiplexed in such a manner that the class with the attached check code is brought to the forefront.
When a terminal device receives multiplexed data of a plurality of transport channels TrCH from a base station in a W-CDMA system compliant with the 3GPP standard, the terminal device decodes a TFCI (Transport Format Indicator) bit that has been mapped to each frame every frame of 10 ms. On the basis of the TFCI bit, the device identifies the bit rate of each transport channel TrCH, i.e., the information bit length per unit time (namely the transport format). On the basis of the identified transport format, the terminal device thenceforth demultiplexes the transport data of each transport channel from the multiplexed data that has been received.
In relation to a channel having a low user rate, there are cases where no TFCI bit exists. At such time, the transport format of each transport channel TrCH is discriminated by BTFD (Blind Transport Format Detection) processing that utilizes a CRC check, and transmit data of each transport channel TrCH is reproduced from the multiplexed data received.
In accordance with the 3GPP standard, BTFD processing is applied when voice code is received. More specifically, in accordance with, e.g., the AMR scheme, a voice codec on the transmitting side expresses a voice signal by {circle around (1)} an LSP parameter expressing the human vocal tract, {circle around (2)} a pitch-period component expressing the periodicity of voice, {circle around (3)} a noise component included in voice, {circle around (4)} gain of the pitch-period component and {circle around (5)} gain of the noise component, extracts each of these elements from input voice, quantizes these elements and outputs the quantized data as voice code. The LSP parameter, pitch-period component and pitch gain are important and hence are assigned to a first transport channel (TrCH of Class A). The noise component and noise gain may contain a small amount of error without critical consequences and therefore are assigned to second and third transport channels (TrCH of Classes B and C).
The transmitting apparatus expresses the voice code of each class obtained from the voice codec by a number of bits conforming to each prescribed bit rate, attaches a fixed-length check code to the voice code of Class A, subjects the voice code of each class to error-correction encoding processing and transmits voice code, which has undergone error-correction encoding processing in each class, upon multiplexing it in such a manner that Class A having the attached check code is brought to the forefront.
The receiving apparatus discriminates the transport format (bit length) of the transport channel TrCH of each of Classes A to C by BTFD processing utilizing the CRC check, extracts the received voice code of each class based upon the bit length and inputs this voice code to a voice codec. The latter reproduces the voice signal from the voice code and outputs the voice signal. On the receiving side, the receive physical channel undergoes a transition to a logical channel in a higher layer. During the transition from the physical channel to the logical channel, a transition is made to the transport channel (TrCH) state. In voice there are three channels of TrCH and one logical channel, and the transport channels are of Class A, Class B, Class C.
FIG. 9 is a block diagram of a mobile station according to the prior art. When a transmission is made, a voice codec 1 converts a voice signal that enters from a microphone 2 to voice code by means of the AMR encoding scheme every transmission time interval TTI of 20 ms and inputs the voice code to a data distributor 4 as voice code of Classes A to C. In accordance with directions from the voice codec, the data distributor 4 selectively inputs the voice code of Classes A to C to transmit buffers 51 to 53 the length of encoding time whereof is 20 ms.
The transmit buffers 51 to 53 write the voice code (transport data) of classes A to C to buffer memories (not shown) every 20 ms and input the data to encoding processors 61 to 63, respectively, which constitute the succeeding stage.
The encoding processors 61 to 63 each encode the 20-ms transport data (after attaching a CRC check bit with regard to Class A) in accordance with convolutional or turbo encoding and input the encoded data to a multiplexer 7 upon dividing it into frame units (units of 10 ms). The multiplexer 7 multiplexes the error-corrected encoded data, which enters from the encoding processors 61 to 63, every 10 ms, creates one frame's worth of multiplexed data and transmits the multiplexed encoded data as in-phase component data.
A control signal generator 8 outputs control data such as a pilot PILO, TFCI and TPC as quadrature-component data at a fixed symbol rate. A QPSK spreader 9a of a QPSK spreader & modulator 9 subjects the entered in-phase component (I-channel component) and quadrature component (Q-channel component) to spread-spectrum modulation using a predetermined spreading code, applies a DA conversion and inputs the resultant signal to a QPSK orthogonal modulator 9b. The latter subjects the I-channel signal and Q-channel signal to QPSK orthogonal modulation, and a radio transmitter 10 frequency-converts (IF→RF) the baseband signal output from the orthogonal modulator to a high-frequency signal, performs high-frequency amplification, etc., and transmits the amplified signal from an antenna ANTT.
At reception, a radio receiver 11 subjects a high-frequency signal received from an antenna ANTR to a frequency conversion (RF→IF conversion) to obtain a baseband signal, after which it subjects the baseband signal to orthogonal detection to generate in-phase component (I component) data and a quadrature component (Q component) data, applies an AD conversion and inputs the results to a despreading demodulator 12. The latter applies despread processing to the I- and Q-component signals using a code identical with that of the spreading code, demodulates (synchronously detects) the sent encoded data and inputs the result to a demultiplexer 13.
The demultiplexer 13 demultiplexes the data of Classes A to C from the input multiplexed data frame by frame and inputs the resultant data to decoding processors 141 to 143, respectively. The processors 141 to 143 each join two items of 10-ms data to form data having a transmission time interval TTI of 20 ms, subsequently subject the data of classes A to C to error-correction decoding processing to decode the original voice code data of classes A to C, respectively, and write the decoded data to buffer memories of receive buffers 151 to 153. The receive buffers 151 to 153 read the voice-code data of Classes A to C out of the buffer memories synchronously and input the data to the data distributor 4. The latter inputs the voice code data of each class to the voice codec 1. The latter reproduces the voice signal from the voice code and outputs the voice signal from a speaker 3.
To summarize the foregoing, a channel codec 21 on the transmitting side of a W-CDMA system accepts voice code data of Classes A to C from a higher layer, executes encoding processing for every transport channel (TrCH) of Classes A to C, multiplexes the encoded data and transmits the data upon mapping it to a physical channel. Conversely, a channel codec 22 on the receiving side demultiplexes data for every transport channel (TrCH) of Classes A to C from multiplexed data on a physical channel and delivers the results (voice code of Classes A to C) to a higher layer. As mentioned above, the voice codec 1 on the transmitting side converts a voice signal that enters from the microphone 2 to voice code by means of the AMR encoding scheme every transmission time interval TTI of 20 ms and inputs the voice code to the data distributor 4 as voice code of Classes A to C. The bit rate (bit length) of the voice code of each class is specified by the base station when a call is connected. That is, when a call is connected, the base station reports a plurality of combination candidates of bit rates of each of the Classes A to C to the originating terminal and terminating terminal and specifies at which bit-rate combination voice code should be transmitted. FIG. 10 is an example of voice formats (candidates of bit-rate combinations) in accordance with the 3GPP standard. This illustrates bit lengths for expressing voice code of Classes A to C encoded every TTI of 20 ms. Ten types of bit-rate combinations are indicated in FIG. 10 and frame-type numbers have been attached to identify the combinations. Classifying the bit-rate combination candidates, we have {circle around (1)} a silence bit-rate combination (1111), {circle around (2)} a background-noise bit-rate combination (0001) and {circle around (3)} voice-activity bit-rate combinations (0000 to 1110). In case of voice activity, which bit-rate combination is to be used is decided by the communication traffic at the time the call is connected. More specifically, when there is no traffic, an exchange of high-quality voice data is performed at a high bit rate of 12.2 kbps. Conversely, when traffic is heavy, a change is made to a low bit rate in accordance with the degree of congestion to reduce the bit length of data sent and received. It should be noted that once the voice-activity bit-rate combination has been decided when the call is connected, this rate is maintained until the call ends, and there is no changeover of bit rate in the interim. Under these circumstances, the voice-activity bit-rate combination is only that of frame type 1110 for 12.2 kbps.
Background noise is necessary in order to impart naturalness in terms of the sense of hearing. In human conversation there exist intervals with speech (voice activity segments) and intervals without speech (silence segments) during which conversation pauses or in which one waits silently for the other party to speak. In general, background noise produced in an office, by vehicles or from the street is superimposed upon speech. In actual voice communication, therefore, there are intervals (voice activity segments) in which background noise is superimposed upon speech, and intervals (silence segments) consisting solely of background noise. This means that a large-scale reduction in amount of transmission can be achieved by detecting silence segments and halting the transmission of information in the silence segments. In silence segments, however, either no particular action is taken or there is no other choice but to output a certain level of noise. This produces an unnatural condition that seems odd to the listener. Accordingly, when a state of silence continues for seven consecutive silence frames, as shown in FIG. 11, one background-noise frame that is necessary to generate background noise is inserted, thereby making possible natural reproduction without strangeness on the receiving side while reducing the quantity of background noise transmitted.
When a call is connected, the base station reports the bit-rate combination candidates of Classes A to C shown in FIG. 10 to the originating and terminating terminals and specifies, by means of the frame-type number, at which bit-rate combination voice code should be transmitted. The voice codec 1 (FIG. 9) of the originating terminal expresses the voice code of each class by the bit length of which it has been instructed by the base station. The channel codec 21 on the transmitting side attaches a CRC check code of fixed length to the voice code of Class A, subjects the voice code of each class to error-correction encoding processing, divides the encoded data into frame units (units of 10 ms), multiplexes the error-corrected encoded data of each class every 10 ms, creates one frame of multiplexed data and transmits the same. The channel codec 22 on the receiving side of the terminating terminal demultiplexes the data of each class from the multiplexed data and applies decoding processing to each item of data.
FIG. 12 shows the data structure of each TTI=20 ms class after demultiplexing, in which (A) is a diagram showing the data structure of Class A and (B), (C) are diagrams showing the data structures of Classes B, C, respectively. The data of Class A is composed of {circle around (1)} a voice code portion A1 of Class A having a bit length that conforms to the frame type indicated by the base station, {circle around (2)} a fixed-length CRC check code portion A2 and {circle around (3)} an empty portion A3. The data of Classes B, C is composed of {circle around (1)} voice code portions B1, C1 having bit lengths that conform to the frame type indicated by the base station, and {circle around (2)} empty portions B2, C2. It should be noted that although no transmission of signals takes place in the empty portions, signals based on noise are transmitted.
The channel codec 22 must accurately extract only the voice code portions A1, B1, C1 (remove signals ascribable to noise) from the data having the structure shown in FIG. 12 and input these portions to the voice codec. Accordingly, in the prior art, demultiplexing is performed upon discriminating the bit lengths of the voice code of each of the classes by BTFD (Blind Transport Format Detection) utilizing a CRC check. Specifically, the channel codec 22 assumes that the voice code of Class A is expressed by the bit counts (0, 39, 42, 49, . . . , 81) per unit time of Class A in each frame type shown in FIG. 10 and applies error-correction decoding processing to the receive data. It should be noted that numbers nend=1, 2, . . . are assigned in order of increasing bit count.
This is followed by investigating whether one decoded result is correct in regard to all patterns by a CRC check applied to the decoded data, after which a search is made for decoded results determined to be correct by the investigation. Reference is then had to FIG. 10 to obtain the bit count per unit time (i.e., the bit rate) of each class in the bit-rate combinations prevailing at this time, and it is determined that this bit count is the bit count that expresses the voice code of each class. If the bit length of each class is determined, the channel codec 22 on the receiving side accurately extracts only the voice code portions from the data having the structures shown in FIGS. 12(A) to (C) and inputs these portions to the voice codec.
BTFD processing will be described below. First, however, convolutional encoding and Viterbi decoding, which are necessary in terms of comprehending BTFD processing, will be described.
FIG. 13 shows an example of a convolutional encoder. The encoder has a 2-bit shift register SFR and two exclusive-OR circuits EXOR1, EXOR2. The EXOR1 outputs the exclusive-OR g0 between an input and R1, and the EXOR2 outputs the exclusive-OR g1 (outputs “1” when “1” is an odd number and “0” otherwise) between the input and R0, R1. Accordingly, the input/output relationship of the convolutional encoder and the state of the shift register SFR in case of input data 01101 are as shown in FIG. 14.
The content of the shift register SFR of the convolutional encoder is defined as the state, and there are four states, namely 00, 01, 10 and 11, as shown in FIG. 15, which are expressed by state a, state b, state c and state d, respectively. With the convolutional encoder of FIG. 13, the outputs (g0, g1) and the next state are uniquely decided depending upon which of the states a to d is indicated by the state of the shift register SFR and depending upon whether the next item of input data is “0” or “1”. FIG. 16 is a diagram showing the relationship between the states of the convolutional encoder and the inputs and outputs thereof, in which the dashed lines indicate a “0” input and the solid lines a “1” input. For example,
(1) if “0” is input in state a, the output is 00 and the state is a; if “1” is input, the output is 11 and the state becomes c;
(2) If “0” is input in state b, the output is 11 and the state is a; if “1” is input, the output is 00 and the state becomes c;
(3) if “0” is input in state c, the output is 01 and the state becomes b; if “1” is input, the output is 10 and the state becomes d; and
(4) if “0” is input in state d, the output is 10 and the state becomes b; if “1” is input, the output is 01 and the state becomes d.
If the convolutional codes of the convolutional encoder shown in FIG. 13 are expressed in the form of a lattice using the above input/output relationship, the result is as shown in FIG. 17(a), where k signifies the time at which a kth bit is input and the initial (k=0) state of the encoder is a(00). The dashed line indicates a “0” input and the solid line a “1” input, and the two numerical values on the lines are the outputs (g0, g1). Accordingly, it will be understood that if “0” is input in the initial state a(00), the output is 00 and the state is state a, and that if “1” is input, the output is 11 and the state becomes state c.
Upon referring to this lattice-like representation, it will be understood that if the original data is 11001, state c is attained via the path indicated by the two-dot dashed line in FIG. 17(b), and that the outputs of the encoder become
11→10→10→11→11
If the ideal error-free state is assumed, in which the receive data (g0, g1) of the decoder is 11→10→10→11→11, a path indicated by the two-dot dashed line shown in FIG. 18(a) is obtained. By making the dashed lines “0”s and the solid lines “1”s, the decoded result 11001 can be obtained, as illustrated in FIG. 18(b). In actuality, however, there are many cases where the receive data contains an error. If the fifth bit develops an error so that hard-decision receive data (g0, g1) is 11→10→00→11→11, as shown in FIG. 18(c), confusion occurs at data-input time k=2 as to whether to branch to 10 or 01 (error count ERR=1). If 10 is construed to be the state and the upper path is selected, state c is reached without confusion at k=3 and k=4. Accordingly, the error count becomes error count ERR=1 on the path of the two-dot dashed line and the decoded result at this time becomes 11001. On the other hand, if 01 is construed to be the state and the lower path is selected at data-input time k=2, then confusion occurs at time k=3 also as to where to branch and total error count ERR=2 is the result. Thereafter, and in similar fashion, paths are selected and, when branching confusion occurs, ERR is counted up. The following results are eventually obtained:                total error count ERR when decoded result is 11001: 1        total error count ERR when decoded result is 11100: 2        total error count ERR when decoded result is 11110: 3        total error count ERR when decoded result is 11111: 4Accordingly, the decoded result 11001 for which the error count ERR is smallest is selected and output. If this arrangement is adopted, the original data 11001 can be reconstructed correctly even if the receive data is erroneous.        
Processing for thus obtaining the error counts ERR of all possible paths based upon the receive data and decoding the original data from the path for which the error count is smallest is complicated. Accordingly, Viterbi decoding is performed as set forth below. It will be assumed that the receive data is 111000 as the result of a hard decision. At state a where k=3 in FIG. 18, there are two input paths. If only the relevant paths are extracted and drawn, the result is as shown in FIG. 19(a). The two paths are path (1) and path (2) shown in the drawing. If the hamming distances (referred to hereafter as “path metric values”) between the receive data and the decoded data obtained on respective ones of the paths are calculated, the results will be 3 and 4, as illustrated in FIGS. 19(b), (c), respectively.
On the basis of the results of calculation, the path metric value for which the assumption is “state a reached by following path (1)” is smaller than that for which the assumption is “state a reached by following path (2)”. Accordingly, since path (1) has a high reliability in that it is the path conforming to the data transmitted, this path is left as a survivor and the other path is discarded. If this processing for adopting or rejecting paths is executed successively with regard to each of the states a to d starting from time k=1, it is possible to find the paths for which the path metric values to reach each of the states a, b, c, d at any time k are smallest (the paths of minimum error). Similar adopt-or-reject processing can continue from this point onward.
Thus, when N-items of receive data have been input, the path for which the path metric value is smallest is decided from among the four paths of minimum path metric value (minimum error) leading to respective ones of the states a, b, c, d at k=N, and the decoded data is output based upon this path. FIG. 20 illustrates the shortest paths leading to respective ones of the states a to d at each time k (=1 to 5) when the receive data is 11 10 00 11 11. The numerical values on the lines are the path metric values. At data-input time k=5, the path metric value of the path to state c is the smallest. Accordingly, if trace-back processing is executed along this path from state c at time k=5, data 11001 will be obtained. This is the decoded data. The above decoding algorithm is the Viterbi algorithm.
It should be noted that if we write a0(nend) for the path metric value of state a (=state 0) at data-input time k=nend, write amax(nend) for the maximum path metric value among the states a to d and write amin(nend) for the minimum path metric value among the states a to d, then the characteristic will be such that the smaller the error with respect to the encoded data, the more conspicuous the relation a0(nend)>amin(nend) becomes. More specifically, the characteristic is such that the smaller the error with respect to the encoded data, the larger a0(nend) and the smaller amin(nend). Consequently, the ratio of [a0(nend)−amin(nend)] to [amax(nend)−amin(nend)] increases. According to this characteristic, the smaller the error, the smaller the value of S(nend), which is given by the following equation:S(nend)=−10 log [{a0(nend)−amin(nend)}/{amax(nend)−amin(nend)}] [dB]  (1)In BTFD processing, the S(nend) value is used. BTFD processing will be described in detail in accordance with FIG. 21. In outline, however, the following processing is executed in stages:
(a) A plurality bit-rate candidates is specified.
(b) Viterbi decoding is performed in order of increasing bit rate (increasing nend) with regard to the bit rate of class A in each bit-rate candidate, Add-Compare-Select (referred to as “ACS” below) processing is executed, path metric values are found and S(nend) is calculated in accordance with Equation (1) using these path metric values.
(c) S(nend) and a threshold value D are judged in terms of their magnitudes;
(d) If S(nend) is equal to or less than the threshold value D, trace-back processing is executed from the state in which the path metric value is smallest at the final bit position.
(e) A CRC check is applied to decoded data obtained by trace-back processing.
(f) If the CRC check is acceptable, the present S(nend) and Smin, which is the smallest value thus far, are compared.
Steps (b) to (f) are executed a number of times equivalent to the number of bit-rate candidates and the candidate for which the result of the CRC check is acceptable and, moreover, which eventually is the most reliable, i.e., the candidate for which S(nend) is smallest, is selected. It is decided that the number of bits of each class in this bit-rate candidate is a number of bits that expresses the voice code of each class. If the result of (c) is that S(nend) is greater than the threshold value D, or in other words, if the reliability is low, the processing of (d), (e), (f) is not executed.
FIG. 21 if a flowchart of BTFD processing.
Since bit-rate candidates (FIG. 10) are specified from within a host application, numbers are assigned in order of increasing bit rate (number of bits per unit time) of Class A in the manner nend=1, 2, 3, . . . (step 101). Next, initialization is performed as follows: nend=1, Smin=D, nend′=0 (step 102).
Thereafter, ACS processing is executed up to the nend position (step 103) and S(nend) is calculated in accordance with Equation (1) (step 104). If S(nend) has been obtained, then this S(nend) and the threshold value D are compared (step 105).
If S(nend)≦D holds, trace-back processing is executed from the state in which the path metric value at the nend position is smallest (step 106). A CRC check is applied to the decoded data obtained by trace-back processing (step 107). If the CRC check is OK, then the present S(nend) and Smin, which is the minimum value thus fare, are compared (step 109).
If Smin>S(nend) holds, the minimum value is updated in the form Smin=S(nend) and nend at this time, namely the number of bits of the voice code in Class A, is stored in the form nend′=nend (step 110). It is then determined whether nend is the final candidate (step 111). If it is not the final candidate, nend is incremented by the operation nend=nend+1 (step 112) and processing from step 103 onward is executed.
If S(nend)>D is found to hold at step 105, or if a “NO” decision is rendered by the CRC check at step 108, or if Smin≦S(nend) is found to hold at step 109, then the processing of step 111 is executed.
If the above processing has been repeated with regard to all candidates nend, a “YES” decision will be rendered at step 111 and then it is determined whether nend′=0 holds (step 113). If the decision is “YES”, an error is output (step 114). If nend′=0 does not hold, however, it is judged that nend′ is the bit rate (number of bits) of Class A in the most reliable bit-rate combination and this is output (step 115). This is followed by referring to the bit-rate combination candidates of FIG. 10 to find the bit rates (numbers of bits) of the other classes.
FIG. 22 is a block diagram of a channel codec on the receiving side for executing BTFD processing. A separator/combiner (not shown) combines, on a per-class basis, data that has been separated on a per-class basis from multiplexed data, thereby creating data of each class, the TTI of the data being 20 ms. A receive-data memory 22a receives and holds the data of each class, where TTI=20 ms holds. A Viterbi decoder 22b includes an ACS an ACS operation/path-metric memory/path memory unit 31, a trace-back unit 32 and a post-trace-back memory 33. The path memory stores, at each time k, each of four paths, for which the path metric values are smallest, leading to respective ones of the states a, b, c, d, and the path-metric memory stores the path metric value of each path. In the example where k={circle around (5)} holds in FIG. 20, we have the following:
path leading to state a is 111000 and path metric value is 2;
path leading to state b is 111110 and path metric value is 3;
path leading to state c is 11001 and path metric value is 1; and
path leading to state d is 111111 and path metric value is 3.
The trace-back unit 32 decides the path for which the path metric value is smallest from among the four paths leading to the states a, b, c, d, executes trace-back processing along this path, obtains the decoded data and stores it in the post-trace-back memory 33.
With regard to Class A, the Viterbi decoder applies Viterbi decoding processing to data up to the nend position. With regard to Classes B, C, the data lengths are not known and therefore the Viterbi decoder executes Viterbi decoding processing up to the tail-end position, stores in the path memory, at each time k corresponding to each data-length candidate, four paths, for which the path metric values are smallest, leading to respective ones of the states a, b, c, d, and stores each of the path metric values in the path-metric memory.
A CRC operation unit 22c performs a CRC check operation based upon the result of decoding Class A. A post-CRC check memory 22d corresponds to the receive buffer 15 in FIG. 9 and stores decoded data (voice code) of Class A prevailing when the CRC check is OK and, moreover, Smin holds. Further, the trace-back unit obtains the path for which the path metric value is smallest from among the four paths at time k corresponding to the bit counts of Classes B, C decided by BTFD processing, executes trace-back processing along this path, acquires the voice codes of Classes B, C and stores them in the post-CRC check memory 22d. 
A BTFD controller 22e executes BTFD processing in accordance with the flowchart of FIG. 21 and decides the bit count of the voice code of each class. A candidate-rate setting/holding unit 34 holds a plurality of bit-rate candidates reported from a higher layer (host application) 41 and sets prescribed bit rates in the receive-data memory 22a. 
The higher layer (host application) 41 reports the candidate rate information (the bit lengths of Classes A, B, C shown in FIG. 10) to the BTFD controller 22e in advance, and the candidate-rate setting/holding unit 34 holds the reported candidate rate information (bit-rate combination information).
Meanwhile, receive data of each class that has been separated by a separator (not shown) is held in the receive-data memory 22a, and the candidate-rate setting/holding unit 34 of BTFD controller 22e sets the bit rates of Class A of the plurality of bit-rate combination candidates in the receive-data memory 22a in order of increasing bit rate.
The receive-data memory 22a that has received the bit rates inputs the receive data of Class A whose numbers of bits conform to the above bit rates to the ACS operation/path-metric memory/path memory unit 31 of the Viterbi decoder. The ACS operation/path-metric memory/path memory unit 31 executes the ACS operation, holds the path metric values, which are the results of this operation, in an internal path-metric memory and reports the maximum path metric value amax (nend), the minimum path metric value amin (nend) and the path metric value a0 (nend) of state a (state 0) to the BTFD controller 22e. 
The BTFD controller 22e calculates S(nend) and compares S(nend) and the threshold value D. If it is judged as a result that trace-back is to be performed, then the BTFD controller 22e inputs trace-back start-up information to the trace-back unit 32. In response, the trace-back unit 32 performs trace-back and stores the decoded results up to the nend position in the post-trace-back memory 33.
The receive-data memory 22a thenceforth inputs, to the Viterbi decoder 22b, the receive data of Classes B, C whose bit counts conform to the bit rates of Classes B, C in the maximum-rate combination (frame type 1110 in the example of FIG. 10). The Viterbi decoder 22b executes Viterbi decoding processing and stores the path obtained in the path memory. That is, as mentioned above, the Viterbi decoder 22b stores, at each time k corresponding to each data-length candidate, four paths, for which the path metric values are smallest, leading to respective ones of the states a, b, c, d, and stores each of the path metric values in the path-metric memory.
If the decoding of Class A is finished, the post-trace-back memory 33 inputs, to the CRC operation unit 22c, decoded results in an amount equivalent to the bit count+number of CRC codes of Class A specified by the bit-rate combination information reported from the BTFD controller 22e. The CRC operation unit 22c performs a CRC check operation and reports the CRC check results to the BTFD controller 22e. On the basis of the CRC check results, the BTFD controller 22e, in accordance with the processing flowchart of FIG. 21, compares Smin and S(nend) and updates the values of Smin, nend′ if Smin>S(nend) holds. If the CRC check is OK and, moreover, the condition Smin>S(nend) holds, then the decoded result from which the CRC check code has been deleted is stored, instead of the decoded result thus far, in the post-CRC check memory 22d. If the BTFD controller 22e thenceforth executes the above processing for the amount of bit-rate combination candidates with regard to Class A, then the controller will recognize that the number of bits conforming to the value of nend′ at this time is the number of bits of the voice code of Class A. It should be noted that voice code data of Class A whose number of bits conforms to the value of nend′ has been stored in the post-CRC check memory 22d previously.
Next, the BTFD controller 22e identifies the numbers of bits of voice code of Classes B, C from bit-rate combination candidate table (FIG. 10) and the numbers of bits of Class A. The BTFD controller 22e thenceforth starts up the trace-back unit 32. The latter obtains, from the path memory, the path for which the path metric value is smallest from among four paths at time k corresponding to the bit lengths of Classes B, C, executes trace-back processing along this path, acquires the voice codes of Classes B, C and stores the voice code in the post-CRC check memory 22d via the CRC operation unit 22c. When the voice codes of Classes A, B, C have been obtained, the post-CRC check memory 22d sends these voice codes to the voice codec 1 as one logical channel, and the voice codec 1 reconstructs the voice signal from the voice codes received.
With conventional BTFD processing in line with the 3GPP standard, Viterbi processing and the CRC check operation are executed in order of greater bit rate, i.e., data length, as is evident from the processing flowchart of FIG. 21, the data length of the voice code of each class of maximum reliability is decided after processing has finally been applied to all bit-rate combination candidates, voice code data of each class is cut from the receive data and this data is input to the voice codec. With this conventional method, however, all rate candidates are processed every time. As a consequence, a problem which arises is that the amount of processing is very great and there is an increase in current consumed.
In the conventional BTFD processing sequence, it is assumed that the CRC check will be acceptable even in data lengths that differ from that of the true voice code. As a result, detection processing is executed with regard to all candidates and bit lengths of the voice codes of each of the classes are decided based upon the best candidate in terms of the characteristics. However, the probability that an acceptable CRC check will occur for data lengths other than the true data length originally transmitted is 2−12, where CRC SIZE=12 bits holds, and the improvement in characteristics achieved by this processing can only be considered to be small.
Further, consider a case where bit-rate combination candidates are of three types, namely for silence, for background noise and for 12.2 k voice data. During a call, the 12.2 k voice data will continue to some extent. Nevertheless, in the conventional sequence, Viterbi decoding processing and the CRC check operation are executed every time in the following order: silence, background noise, 12.2 k voice data (namely in order of increasing rate), and ultimately the fact that the 12.2 k voice data is correct is detected and the data is received. The problem which arises is a large amount of processing and increased consumption of current.
Further, in the conventional BTFD processing sequence, the results of decoding for each class for which Viterbi decoding, trace-back processing and the CRC operation have been completed are stored temporarily in the post-CRC check memory 22d. Thus, according to the conventional BTFD processing sequence, both the post-trace-back memory and post-CRC check memory are required. The problem which arises is a great increase in the number of memories used.